The light transmitting performance of an optical fiber is highly dependent upon the properties of the polymer coating that is applied to the fiber during manufacturing. Typically a dual-layer coating system is used where a soft primary coating is in contact with the glass fiber and a harder secondary coating surrounds the primary coating. The harder secondary coating protects the fiber from damage caused by external forces and allows the fiber to be handled without concern of breakage. The softer primary coating dissipates forces that reach the interior of the coated fiber and prevents them from being transferred to the fiber. The primary coating is especially important in dissipating stresses that arise when the fiber is bent. The influence of bending stresses on the fiber needs to be minimized because bending stresses create local perturbations in the refractive index profile of the glass fiber that act to attenuate the intensity of light guided by the fiber. By dissipating stresses, the primary coating minimizes bend-induced attenuation losses.
The functional requirements of the primary coating impose various performance specifications on the materials that are used as primary coatings. The Young's modulus of the primary coating generally needs to be less than 1 MPa, and preferably less than 0.5 MPa, to provide the necessary force dissipation. The glass transition temperature of the primary coating also needs to be less than 0° C., and preferably less than −20° C., to ensure that the primary coating remains soft when the fiber is subjected to low temperatures, such as those that arise in the field in cold climates. In order to ensure uniform deposition on the glass fiber, the primary coating is applied to the glass fiber in liquid form. From a process throughput viewpoint, it is desirable to form the secondary coating on the primary coating as quickly as possible. The desire for high process throughput necessitates use of a primary liquid composition that reacts or cures quickly to form a solid coating that has sufficient mechanical integrity to support application of the secondary coating. In addition, the tensile strength of the solidified primary coating must be high enough to prevent tearing defects during the fiber draw process or post-draw processing of the coated fiber during cabling and installation. Achieving adequate tensile strength is a particular challenge for the primary coating because tensile strength generally decreases as the modulus of the coating material decreases and the force-dissipating function of the primary coating is best achieved with a low modulus material.
In order to meet the performance requirements, primary coating compositions are usually formulated as mixtures of radiation-curable urethane acrylate oligomers and radiation-curable acrylate-functionalized monomer diluents. Upon exposure to light in the presence of a photoinitiator, the acrylate groups rapidly polymerize to form a crosslinked polymer network, which is further strengthened by hydrogen bonding interactions that occur between urethane groups along the oligomer backbone. By varying the urethane acrylate oligomer, it is possible to form primary coatings having very low modulus values, while maintaining sufficient tensile strength. The drawback to coatings based on radiation-curable urethane acrylate oligomers is cost. The desire to improve the economics of the fiber coating process has motivated a search for alternatives to radiation-curable urethane/acrylate oligomers as constituents in the coating formulation.
Non-radiation-curable thermoplastic hard/soft block co-polymers with urethane groups have been proposed as strength additives to a crosslinked radiation-curable all-acrylic optical fiber coating. A high molecular weight thermoplastic additive with urethane groups is expected to entangle within the acrylic network and hydrogen bonding interactions between urethane groups are expected to provide structural reinforcement to the coating to improve tensile strength. The ability to control tensile strength through the level of loading of urethane-containing thermoplastic strength additives is also expected. However, the strong intermolecular self-interactions in urethane-containing thermoplastic additives, especially in the generally higher molecular weight materials that are commercially available and economical, could limit the amount of urethane-containing additive that can mix homogeneously with most acrylic functional coating components. High loading of high molecular weight urethane-containing thermoplastic additives in the liquid primary coating composition may also significantly increase the viscosity of the composition, which could lead to non-uniform coverage of the primary composition on the glass fiber during the coating application process.
Radiation-curable optical fiber coatings having low modulus values and low glass transition temperatures can be prepared using acrylate-functionalized monomers and/or oligomers without urethane groups (e.g. polyalkylene glycol diacrylates). Coating compositions based on acrylate-functionalized monomers and/or oligomers can be used to prepare fiber coatings with good tensile strength. The modulus of the coating can be controlled through the molecular weight of the acrylate-functionalized monomer or oligomer. For primary coatings, low modulus values are desired and may be achieved by curing coating compositions based on higher molecular weight acrylate-functionalized monomers or oligomers. Higher molecular weight acrylate-functionalized monomers or oligomers also provide the lower glass transition temperatures desired for primary coatings. Higher molecular weight urethane-free, acrylate-functionalized starting materials, however, have limited commercial availability. In order to extend the use of urethane-free acrylate-functionalized monomers and oligomers to primary fiber coatings, it is necessary to have wide commercial availability of urethane-free acrylate-functionalized monomers and oligomers.